1. Field of the Invention
The present invention relates to a data communication system, and more particularly to a data communication system suitable for use in a network of components of an audiovisual system on an automobile.
2. Description of the Prior Art
In recent years, automobile audio systems have been changing from systems which give the user only audio information such as music to systems which allow the user to enjoy both audio and visual information. Systems with both audio and visual capabilities are known as audiovisual systems.
Automobile audiovisual systems on automobiles are composed of many different audio and visual components. The audio components include a cassette tape deck, a radio tuner, a CD (compact disc) player, etc., while the visual components include a TV (television) tuner, a navigation system, etc.. Audio signals produced by some of these components are amplified by an amplifier and reproduced from loudspeakers in the automobile. Video signals produced by other components are displayed on a display unit in the automobile. Today, the audiovisual system components are controlled according to digital technology, i.e., by respective controllers in the form of microcomputers.
The components of an audiovisual system are required to be controlled systematically for systematic operation of the components. To meet this requirement, the controllers of the respective components are connected by a communication bus network, so that control data for the components will be transmitted through the communication bus.
For the convenience of description, either one of the components connected to the communication bus is referred to as a "master unit" for controlling the overall network, while each of the other components is referred to as a "slave unit".
When the master unit transmits communication data to a slave unit for access or the slave unit transmits data back to the master unit, it is necessary that the controllers be identified or specified. For this purpose, the controllers are allotted respective addresses indicating themselves.
In order for the master unit to access the slave units, the master unit has to have the addresses of all the slave units registered in itself. The master unit cannot access any slave unit whose address has not been registered by the master unit. Therefore, any such slave unit may not function even if it is physically or electrically connected to the communication bus.
To avoid the above situation, when a request for registration in the system (hereinafter also referred to as a "connection request" is sent from a slave unit to the master unit, the master unit issues, to the slave unit, the connection information of all the other slave units and the master unit which are connected to the communication bus. Since the connection information varies depending on the operating condition of each of the slave units, each slave unit transmits information for a connection request (hereinafter also referred to as "connection request information") to the master unit in each time period. Having received the connection information from the slave units, the master unit transmits the connection information of all the slave units and the connection information of its own to the slave units.
If there is no response from the master unit to the connection information transmitted from the slave units, the slave controller of each of the slave units determines that the master unit has failed to operate or has been disabled, and enters a low power consumption mode, and stands by in that mode. Thereafter, each slave unit sends a connection request to the master unit in each fixed time period (e.g., of 2 seconds) under the control of a timer in its own slave controller, and enters a normal mode if a response is received from the master unit.
FIG. 39 of the accompanying drawings is a timing chart of connection requests from slave units of a conventional automobile audiovisual system.
It is assumed in FIG. 39 that initially, the master unit is disabled due to a temporary voltage drop, for example, or the user presses a reset switch upon a malfunction of the system at a time t.sub.0. The master unit fails to respond to a connection request R.sub.NA from a slave unit A at a time t.sub.2 and a connection request R.sub.NB from a slave unit B at a time t.sub.1. Thereafter, the slave units A, B enter a low power consumption mode. Then, the slave units A, B temporarily enter a normal mode and send respective connection requests R.sub.NA, R.sub.NB to the master unit in each fixed time period (e.g., of 2 seconds) under the control of timers in their own slave controllers. If no response comes back from the master unit, the slave units A, B enter the low power consumption mode again.
When the master unit is subsequently enabled at a time t.sub.3, its master controller waits for connection requests from the slave units A, B for a wait time period (e.g., of 1 second) from the time t.sub.3 to a time t.sub.4. Since the slave controllers of the slave units A, B send connection requests R.sub.NA, R.sub.NB every 2 seconds, only the slave controller of the slave unit B can successfully request a connection during the wait time period from the time t.sub.3 to the time t.sub.4. Consequently, the slave unit A is unable to enter the normal mode until it sends a next connection request R.sub.NA at a time t.sub.5.
With the communication bus of the conventional automobile audiovisual system, therefore, even when the master unit is activated again from a disabled state, some of the slave units may be delayed in resuming their normal operation because the slave units issue connection requests at different times, as described above. The delayed operation of some slave units may be inconvenient for the user of the automobile audiovisual system.
One solution would be to increase the wait time period for the master unit to wait for connection confirmation requests after the master unit is enabled. However, inasmuch as the period of time required for the entire automobile audiovisual system to be enabled would also be increased, the automobile audiovisual system might also be inconvenient for the user.
FIG. 40 of the accompanying drawings is a timing chart of an exchange of connection requests and connection information between master and slave units connected through a communication bus of another conventional automobile audiovisual system.
When a slave unit A sends a connection request at a time t.sub.1, the master unit transmits the connection information of its own and of all slave units controlled by the master unit to the slave unit A a time period .DELTA.t after the time t.sub.1. Subsequently, the slave unit A sends a connection request in each time period (e.g., of 2 seconds), and the master unit transmits the above connection information to the slave unit A in response to each such connection request.
Concurrent with this, the master unit determines whether there have been connection requests from all the slave units within a certain time period (e.g., of 5 seconds), for thereby checking the number and types of the connected slave units to confirm any slave units that have been disconnected or dropped from service or that have been newly added. For example, if a slave unit B which has been connected so far is disconnected or dropped from service at a time t.sub.2, then the master unit receives no connection request from the slave unit B at a time t.sub.3, when the master unit confirms the number of connected slave units. Thereafter, the master unit regards the slave unit B as being disconnected, and subsequently transmits the connection information of its own and of all the slave units except the slave unit B. Conversely, when the slave unit A is newly connected and requests a connection at the time t.sub.1 the master unit determines that the slave unit A has not requested a connection so far, and subsequently transmits the connection information of its own and of all the slave units including the slave unit A.
However, the other conventional automobile audiovisual system is required to effect a frequent exchange of data for establishing a connection between the master and slave units. Since the communication bus is occupied for a long period of time by the exchange of the data for establishing a connection, the audiovisual data cannot efficiently be transmitted in the automobile audiovisual system.
In the conventional automobile audiovisual systems, the controllers of the master and slave units are allotted respective communication addresses of their own. Consequently, in the case where a plurality of slave units, such as CD players, having identical slave controllers and designed to perform identical functions, when the user tries to operate on one of the CD players, both of the CD players would be caused to operate in an identical fashion. To avoid such a shortcoming, the user is required to change the setting of a switch, such as a DIP switch, which is mounted on each slave unit for setting a communication address, to assign a different communication address to one of the CD players. However, the manual address setting procedure is time-consuming and is not reliable for setting a proper communication address.